The present invention relates to well logging, and more particularly to a communication system for measuring characteristics of earth formations in a borehole using fiber optic logging cables.
As an oil well is being drilled, a sonde is usually lowered periodically into the borehole to measure characteristics of the earth formations it traverses. Power, communication and control instructions, and data are then relayed between the sonde and instrumentation and control facilities at the surface of the earth through a logging cable which supports and moves the sonde within the borehole. As measurements and measuring instruments have become more sophisticated, data transmission rates through the cable have increased to the point where existing cables can become saturated.
Fiber optic technology can increase data transmission rates several orders of magnitude, and fiber optic cable logging systems for this purpose have been proposed. Since the cable is wound onto and off a rotating winch drum at the surface as the sonde is raised and lowered in the borehole, these prior art devices have coupled the optical signals through optical slip rings to stationary signal processing equipment at the surface (adjacent the wellhead).
Due to the extreme precision required for optical slip rings, such systems are not ideal for well logging applications. The extremely small optical fibers must be maintained in exact alignment for efficient light energy coupling. However, the winch drums and cables typically weigh several tons. During transportation from one well to another, the equipment must withstand many miles of abuse in hostile terrain. During the logging operation, the winch drum may pull upon the logging cable with thousands of pounds of force. To meet these demands, the journals supporting the winch drum are of a size and design wholly inappropriate to the extreme precision required for optical slip rings. Journals which could also meet such optical requirements would be prohibitively expensive.
A need therefore remains for a well logging fiber optic communication system which can inexpensively, effectively and reliably couple optical information between a fiber optic cable and the control and information processing facilities outside the winch drum at the top of the borehole.